In hospitalized patients Staphylococcus aureus is a major cause of infections associated with indwelling medical devices, such as catheters and prostheses, and related infections of surgical wounds. A significant increase in Staphylococcus aureus isolates that exhibit resistance to most known antibiotics has been observed in hospitals throughout the world. The recent emergence of resistance to vancomycin, the last remaining antibiotic for treating methicillin-resistant Staphylococcus aureus (MRSA) infections, has emphasized the need for alternative prophylactic or vaccine strategies to reduce the risk of nosocomial S. aureus infections.
Initial localized infections of wounds or indwelling medical devices can lead to serious invasive infections such as septicemia, osteomyelitis, and endocarditis. In infections associated with medical devices, plastic and metal surfaces become coated with host plasma and extracellular matrix proteins such as fibrinogen and fibronectin shortly after implantation. The ability of S. aureus to adhere to these proteins is of crucial importance for initiating infection. Vascular grafts, intravenous catheters, artificial heart valves, and cardiac assist devices are thrombogenic and prone to bacterial colonization. S. aureus is the most damaging pathogen to cause such infections.
Fibrin is the major component of blood clots, and fibrinogen/fibrin is one of the major plasma proteins deposited on implanted biomaterials. Considerable evidence exists to suggest that bacterial adherence to fibrinogen/fibrin is important in the initiation of device-related infection. For example, as shown by Vaudaux et al., S. aureus adheres to in vitro plastic that has been coated with fibrinogen in a dose-dependent manner (J. Infect. Dis. 160:865-875 (1989)). In addition, in a model that mimics a blood clot or damage to a heart valve, Herrmann et al. demonstrated that S. aureus binds avidly via a fibrinogen bridge to platelets adhering to surfaces (J. Infect. Dis. 167: 312-322 (1993)). S. aureus can adhere directly to fibrinogen in blood clots formed in vitro, and can adhere to cultured endothelial cells via fibrinogen deposited from plasma acting as a bridge (Moreillon et al., Infect. Immun. 63:4738-4743 (1995); Cheung et al., J. Clin. Invest. 87:2236-2245 (1991)). As shown by Vaudaux et al. and Moreillon et al., mutants defective in the fibrinogen-binding protein clumping factor (ClfA) exhibit reduced adherence to fibrinogen in vitro, to explanted catheters, to blood clots, and to damaged heart valves in the rat model for endocarditis (Vaudaux et al., Infect. Immun. 63:585-590 (1995); Moreillon et al., Infect. Immun. 63: 4738-4743 (1995)).
An adhesin for fibrinogen, often referred to as “clumping factor,” is located on the surface of S. aureus cells. The interaction between the clumping factor on bacteria and fibrinogen in solution results in the instantaneous clumping of bacterial cells. The binding site on fibrinogen is located in the C-terminus of the gamma chain of the dimeric fibrinogen glycoprotein. The affinity is very high and clumping occurs in low concentrations of fibrinogen. Scientists have recently shown that clumping factor also promotes adherence to solid phase fibrinogen, to blood clots, and to damaged heart valves (McDevitt et al., Mol. Microbiol. 11: 237-248 (1994); Vaudaux et al., Infect. Immun. 63:585-590 (1995); Moreillon et al., Infect. Immun. 63: 4738-4743 (1995)).
The gene for a clumping factor protein, designated ClfA, has been cloned, sequenced and analyzed in detail at the molecular level (McDevitt et al., Mol. Microbiol. 11: 237-248 (1994); McDevitt et al., Mol. Microbiol. 16:895-907 (1995)). The predicted protein is composed of 933 amino acids. A signal sequence of 39 residues occurs at the N-terminus followed by a 520 residue region (region A), which contains the fibrinogen binding domain. A 308 residue region (region R), composed of 154 repeats of the dipeptide serine-aspartate, follows. The R region sequence is encoded by the 18 basepair repeat GAYTCNGAYT CNGAYAGY (SEQ ID NO: 9) in which Y equals pyrimidines and N equals any base. The C-terminus of ClfA has features present in many surface proteins of Gram-positive bacteria such as an LPDTG (SEQ ID NO: 10) motif, which is responsible for anchoring the protein to the cell wall, a membrane anchor, and positive charged residues at the extreme C-terminus.
The platelet integrin alpha IIbβ3 recognizes the C-terminus of the gamma chain of fibrinogen. This is a crucial event in the initiation of blood clotting during coagulation. ClfA and alpha IIbβ3 appear to recognize precisely the same sites on fibrinogen gamma chain because ClfA can block platelet aggregation, and a peptide corresponding to the C-terminus of the gamma chain (198-411) can block both the integrin and ClfA interacting with fibrinogen (McDevitt et al., Eur. J. Biochem. 247:416-424 (1997)). The fibrinogen binding site of alpha IIbβ3 is close to, or overlaps, a Ca2+ binding determinant referred to as an “EF hand”. ClfA region A carries several EF hand-like motifs. A concentration of Ca2+ in the range of 3-5 mM blocks these ClfA-fibrinogen interactions and changes the secondary structure of the ClfA protein. Mutations affecting the ClfA EF hand reduce or prevent interactions with fibrinogen. Ca2+ and the fibrinogen gamma chain seem to bind to the same, or to overlapping, sites in ClfA region A.
The alpha chain of the leucocyte integrin, alpha Mβ2, has an insertion of 200 amino acids (A or I domain) which is responsible for ligand binding activities. A novel metal ion-dependent adhesion site (MIDAS) motif in the I domain is required for ligand binding. Among the ligands recognized is fibrinogen. The binding site on fibrinogen is in the gamma chain (residues 190-202). It was recently reported that Candida albicans has a surface protein, alpha Intlp, having properties reminiscent of eukaryotic integrins. The surface protein has amino acid sequence homology with the I domain of alpha Mβ2, including the MIDAS motif. Furthermore, alpha Intlp binds to fibrinogen.
ClfA region A also exhibits some degree of sequence homology with alpha Intlp. Examination of the ClfA region A sequence has revealed a potential MIDAS motif Mutations in supposed cation coordinating residues in the DXSXS (SEQ ID NO: 13) portion of the MIDAS motif in ClfA results in a significant reduction in fibrinogen binding. A peptide corresponding to the gamma-chain binding site for alpha Mβ2 (190-202) has been shown by O′Connell et al. to inhibit ClfA-fibrinogen interactions (O′Connell et al., J. Biol. Chem., in press). Thus it appears that ClfA can bind to the gamma-chain of fibrinogen at two separate sites. The ligand binding sites on ClfA are similar to those employed by eukaryotic integrins and involve divalent cation binding EF-hand and MIDAS motifs.
Scientists have recently shown that S. aureus expresses proteins other than ClfA that may bind fibrinogen (Boden and Flock, Mol. Microbiol. 12:599-606 (1994)). One of these proteins is probably the same as the broad spectrum ligand-binding protein reported by Homonylo-McGavin et al., Infect. Immun. 61:2479-2485 (1993). Another is coagulase, as reported by Boden and Flock, Infect. Immun. 57:2358-2363 (1989), a predominantly extracellular protein that activates the plasma clotting activity of prothrombin. Coagulase binds prothrombin at its N-terminus and also interacts with soluble fibrinogen at its C-terminus Cheung et al., Infect. Immun. 63:1914-1920 (1995) have described a variant of coagulase that binds fibrinogen. There is some evidence that coagulase can contribute, in a minor way, to the ability of S. aureus cells to bind fibrinogen. As shown by Wolz et al., Infect. Immun. 64:3142-3147 (1996), in an agr regulatory mutant, where coagulase is expressed at a high level, coagulase appears to contribute to the binding of soluble fibrinogen to bacterial cells. Also, as shown by Dickinson et al., Infect. Immun. 63:3143-3150 (1995), coagulase contributes in a minor way to the attachment of S. aureus to plasma-coated surfaces under flow. However, it is clear that clumping factor ClfA is the major surface-located fibrinogen-binding protein responsible for bacterial attachment to immobilized fibrinogen/fibrin.
The identification and isolation of additional S. aureus extracellular matrix binding proteins would be useful for the development of therapies, diagnosis, prevention strategies and research tools for S. aureus infection.
Accordingly it is an object of the present invention to provide 15 isolated cell-wall associated extracellular matrix-binding proteins of S. aureus and active fragments thereof.
It is a further object of the invention to provide methods for preventing, diagnosing, treating or monitoring the progress of therapy for bacterial infections caused by S. aureus. 
It is a further object of the present invention to provide isolated S. aureus surface proteins that are related in amino acid sequence to ClfA and are able to promote adhesion to the extracellular matrix or host cells.
It is another object of the present invention to generate antisera and antibodies to cell-wall associated extracellular matrix-binding proteins of S. aureus, or active fragments thereof.
It is a further object of the present invention to provide S. aureus vaccines, including a DNA vaccine.
It is a further object of the present invention to provide improved materials and methods for detecting and differentiating S. aureus organisms in clinical and laboratory settings.
It is a further object of the invention to provide nucleic acid probes and primers specific for S. aureus. 
It is a further object of the invention to provide isolated extracellular matrix-binding proteins or peptides of S. aureus. 